Business Combinations - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended		165 Months Ended
	Sep. 30, 2015 USD ($) Entity	Sep. 30, 2014 USD ($) Entity	Sep. 30, 2015 USD ($) Entity Employees	Sep. 30, 2014 USD ($) Entity	Sep. 30, 2015 USD ($) Entity
Business Acquisition [Line Items]
Accretion of the discount on acquisition	$ 3.7	$ 3.9	$ 12.2	$ 10.6
Income related to net adjustments to estimated fair value of liability for earnout obligations	$ 6.0	$ 0.6	$ 13.7	$ 6.0
Number of companies acquired | Entity	35	17	72	49	368
Aggregate amount of maximum earnout obligations related to acquisitions			$ 580.8
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet			223.6
Goodwill	$ 245.8		245.8		$ 245.8
Expiration lists	233.3		233.3		233.3
Non-compete agreements	2.0		2.0		2.0
Total revenues related to acquisitions in the aggregate	1,461.7	$ 1,332.5	4,133.3	$ 3,518.2
Brokerage and Risk Management [Member]
Business Acquisition [Line Items]
Goodwill	245.8		245.8		245.8
Expiration lists	233.3		233.3		233.3
Non-compete agreements	2.0		$ 2.0		2.0
Expiration lists, amortization period, minimum, years			3 years
Expiration lists, amortization period, maximum, years			15 years
Non-compete agreements, amortization period, minimum, years			3 years
Non-compete agreements, amortization period, maximum, years			5 years
Trade names, amortization period, minimum ,years			5 years
Trade names, amortization period, maximum, years			15 years
Write-off of amortizable intangible assets		$ 0.6	$ 0.5	$ 1.2
Maximum [Member]
Business Acquisition [Line Items]
Revenue growth rate			12.00%
2015 Acquisitions [Member] | Market Approach Valuation Technique [Member] | Minimum [Member]
Business Acquisition [Line Items]
Revenue growth rate			4.00%
Discount rate			8.00%
2015 Acquisitions [Member] | Market Approach Valuation Technique [Member] | Maximum [Member]
Business Acquisition [Line Items]
Discount rate			8.50%
2015 Acquisitions [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]
Business Acquisition [Line Items]
Revenue growth rate			5.00%
Discount rate			13.00%
Attrition rate			13.00%
2014 Acquisitions [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]
Business Acquisition [Line Items]
Revenue growth rate			2.00%
Discount rate			11.00%
Attrition rate			3.00%
Business Acquisition [Member]
Business Acquisition [Line Items]
Annualized revenue of business acquisitions			$ 184.5
Total revenues related to acquisitions in the aggregate			67.4
Net earnings			5.1
Business Acquisition [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Expiration lists	233.3		233.3		233.3
Non-compete agreements	2.0		2.0		2.0
Deferred tax liability	7.2		7.2		7.2
Goodwill related to nondeductible amortizable intangible assets			7.2
Business Acquisition [Member] | Expiration Lists [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes	23.2		23.2		23.2
Business Acquisition [Member] | Non-Compete Agreements [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes	0.6		0.6		0.6
William Gallagher Associates Insurance Brokers, Inc. (WGA) [Member]
Business Acquisition [Line Items]
Cash and common stock paid for acquisition			151.5
Annualized revenue of business acquisitions			$ 50.0
Number of employees at office of acquired entity | Employees			200
Goodwill	86.8		$ 86.8		86.8
Expiration lists	62.7		62.7		62.7
Non-compete agreements	$ 0.3		$ 0.3		$ 0.3